Thermal protection systems (TPS) are critical towards making hypersonic flight a reality, as well as, ensuring that future spacecraft will have the capabilities needed to descent through the Martian or other planet atmosphere. There are various passive and active methods of cooling the skin of a vehicle, as well as novel materials, which include CMC's, refractory metals, and ablatives. Regarding ablation technology, having a device that can measure ablation recession rate for real-time, in-flight sensing is highly desirable as progress is made to increasingly prove-out hypersonic technologies through ground testing and eventually flight testing. Such a recession rate sensor would be an important part of an entire sensor suite that helps monitor the structural health of future hypersonic flight vehicles, which of course include both earth based high speed air breathers and spacecraft modules destined for use in NASA's mission to land on Mars and beyond.
Various researchers have investigated different approaches to measuring TPS recession rates that have involved inserting breakwires into the ablative (1Hycal Engineering, “In-Depth Ablative Plug Transducers,” Series #S-2835, 9650, 1992, Telstar Avenue, P.O. Box 5488, El Monte, Calif.), implanting quartz fibers terminating at known depths into the ablator (Legendre, P. J., “Reentry Vehicle Nosetip Instrumentation,” Proceedings of the 22nd International Instrumentation Symposium, San Diego, Calif., 1975.), or embedding a ladder or continuous configuration of resistive elements (Gramer, D. J., Taagen, T. J., and Vermaak, A. G., “Embedded Sensors for Measuring Surface Regression,” NASA Tech Briefs, July 2006.). Another approach employs a capacitive sensor placed in series with an inductor and resistor to form an RLC terminator to a waveguide (4Noffz, G. K., and Bowman, M. P., “Design and Laboratory Validation of a Capacitive Sensor for Measuring the Recession of a Thin-Layered Ablator,” NASA Technical Memorandum 4777, 1996.). This approach is dependent on the material's dielectric properties and may not be applicable for all ablative materials. The entire contents of each of the references discussed above is hereby incorporated by reference.
Current state of the art technology for measuring ablation rate sensors involve approaches that:
are intrusive, thereby affecting the ablator's integrity and requiring the embedded sensor to withstand extremely high temperatures;
rely on sensing changes in ablator material properties, (e.g. sound speed) to detect a recession rate and are therefore sensitive to temperature effects on these same properties;
have relatively poor resolution, thus making it difficult to measure the ablation rate of relatively thinner TPS systems found on booster vehicles.
cannot survive the high temperature environment.
are not conducive for flight applications, with bulky hardware and complex electronics.
There is no practical sensor technology currently in use to measure the recession of an ablative material in-situ and in-flight. The closest is the implementation of breakwires into the ablative as discrete indicators of recession. However, this is an intrusive method, and the desire is strong to have a non-intrusive method developed so that the ablative material is not compromised in any way.